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Checks

One of the primary roles of the agent is the management of system and application level health checks. A health check is considered to be application level if it associated with a service. A check is defined in a configuration file, or added at runtime over the HTTP interface.

There are two different kinds of checks:

• Script + Interval - These checks depend on invoking an external application which does the health check and exits with an appropriate exit code, potentially generating some output. A script is paired with an invocation interval (e.g. every 30 seconds). This is similar to the Nagios plugin system.

• TTL - These checks retain their last known state for a given TTL. The state of the check must be updated periodically over the HTTP interface. If an external system fails to update the status within a given TTL, the check is set to the failed state. This mechanism is used to allow an application to directly report it's health. For example, a web app can periodically curl the endpoint, and if the app fails, then the TTL will expire and the health check enters a critical state. This is conceptually similar to a dead man's switch.

Check Definition

A check definition that is a script looks like:

{
    "check": {
        "id": "mem-util",
        "name": "Memory utilization",
        "script": "/usr/local/bin/check_mem.py",
        "interval": "10s"
    }
}

A TTL based check is very similar:

{
    "check": {
        "id": "web-app",
        "name": "Web App Status",
        "notes": "Web app does a curl internally every 10 seconds",
        "ttl": "30s"
    }
}

Both types of definitions must include a name, and may optionally provide an id and notes field. The id is set to the name if not provided. It is required that all checks have a unique ID, so if names might conflict, then unique ID's should be provided.

The notes field is opaque to Consul, but may be used for human readable descriptions. The field is set to any output that a script generates, and similarly the TTL update hooks can update the notes as well.

To configure a check, either provide it as a -config-file option to the agent, or place it inside the -config-dir of the agent. The file must end in the ".json" extension to be loaded by Consul. Check definitions can also be updated by sending a SIGHUP to the agent. Alternatively, the check can be registered dynamically using the HTTP API.

Check Scripts

A check script is generally free to do anything to determine the status of the check. The only limitations placed are the exit codes must convey a specific meaning. Specifically:

• Exit code 0 - Check is passing
• Exit code 1 - Check is warning
• Any other code - Check is failing

This is the only convention that Consul depends on. Any output of the script will be captured and stored in the notes field so that it can be viewed by human operators.